Sulfur Containing Alpha-Alumina Coated Cutting Tool

ABSTRACT

Cutting tool insert has a substrate and a coating of one or more refractory layers of which at least one layer is an α-Al 2 O 3  layer having thickness of 1 to 25 μm, sulphur content of more than 100 ppm analysed by Secondary Ion Mass Spectroscopy (SIMS), and texture coefficient TC (0 0 12)&gt;4 for the (0 0 12) growth direction. 
     The at least one α-Al 2 O 3  layer is deposited by chemical vapour deposition (CVD) using reaction gases comprising H 2 , CO 2 , AlCl 3  and X, with X being H 2 S, SO 2 , SF 6 , or combinations thereof, and optional additions of N 2  and Ar. The amount of X is at least 1.0 vol-% of the total volume of gases in the reaction chamber. The volume ratio of CO 2  and X in the reaction chamber lies within the range of 1≦CO 2 /X≦7 during deposition of the at least one α-Al 2 O 3  layer.

FIELD OF THE INVENTION

The present invention relates to a cutting tool insert consisting of asubstrate of cemented carbide, cermet, ceramics, steel or a superhardmaterial such as cubic boron nitride (CBN) and a hard coating consistingof one or more refractory layers of which at least one layer is anα-Al₂O₃ layer containing sulphur and having a specified growthorientation defined by the texture coefficient, and a method ofmanufacturing the cutting tool insert.

BACKGROUND OF THE INVENTION

The early approaches to deposit Al₂O₃ on a substrate surface in a CVDprocess based on a AlCl₃/CO₂/H₂ reaction gas mixture had a very lowdeposition rates in the order of about 0.2 μm/h on flat surfaces.Besides the fact that it was not possible to control the phase content,this early Al₂O₃ deposition process also suffered from a pronounceddog-bone-effect, i.e. the deposition rate was higher on the edges thanon the flat surfaces of the substrate. U.S. Pat. No. 4,619,866 by Smithand Lindstöm discloses that dopants, such as H₂S, could be used as acatalyst both to enhance the overall deposition rate but also tosuppress the dog-bone effect. With the introduction of H₂S as a catalystfor the α-Al₂O₃ deposition process the deposition rate increased by afactor of about five coinciding with a more or less complete eliminationof the dog-bone effect as compared to the process without any H₂Spresent

Several attempts have been made to deposit industrial alpha and gammaalumina coatings onto cutting tools by CVD or PVD using sulphurcontaining dopants. In EP-A-0 045 291 the addition of 0.02-0.3 vol-% ofsulphur, selenium or tellurium containing gas, preferably H₂S, to thedeposition gas in the CVD process has been found to increase the growthrate and to improve the uniformity of alumina coatings. EP-A-1 788 124describes the deposition of alpha alumina coatings having a definedcrystal grain boundary orientation wherein the deposition of the aluminais performed by CVD adding from 0.25-0.6 vol-% of H₂S to the depositiongas. EP-A-1 683 893 describes the deposition of alpha alumina coatingshaving a defined amount of Σ3 type grain boundary length wherein thedeposition of the alumina is performed by CVD adding from 1.5-5 vol-%HCl and from 0.05-0.2 vol-% of H₂S to the deposition gas. The prior artliterature does not describe the actual sulphur content in the alphaalumina coatings. Since H₂S has only been used to enhance the growthrate and prevent the dog-bone effect, there have been no attempts to usehigher amounts of sulfur-containing dopants or consider the sulfurcontent in the α-Al₂O₃ coatings in general. One reason for this is that,as disclosed in U.S. Pat. No. 4,619,866, the effect of H₂S on the growthrate on alumina was found to be at maximum at a H₂S concentration of0.25 to 0.3 vol %. Larger amounts of H₂S than about 0.3 vol % were foundto result in strongly reduced growth rates”

OBJECT OF THE INVENTION

It is an object of the present invention is to provide a coated cuttingtool having an α-Al₂O₃ layer that exhibits improved cutting properties,improved chipping resistance and improved crater wear resistance as wellas lower friction in contact with the workpiece over the prior-art.

DESCRIPTION OF THE INVENTION

The present invention relates to a cutting tool insert consisting of asubstrate of cemented carbide, cermet, ceramics, steel or a superhardmaterial such as cubic boron nitride (CBN) and a coating with a totalthickness of 5 to 40 μm, the coating consisting of one or morerefractory layers of which at least one layer is an α-Al₂O₃ layer havinga thickness of 1 to 25 μm, wherein the at least one α-Al₂O₃ layer havinga sulphur content of more than 100 ppm analysed by Secondary Ion MassSpectroscopy (SIMS) and the at least one α-Al₂O₃ layer having a texturecoefficient TC (0 0 12)>4 for the (0 0 12) growth direction, the TC (0 012) being defined as follows:

${{TC}\; \left( {0\mspace{11mu} 0\mspace{11mu} 12} \right)} = {\frac{I\left( {0\mspace{11mu} 0\mspace{11mu} 12} \right)}{I_{0}\left( {0\mspace{11mu} 0\mspace{11mu} 12} \right)}\left\lbrack {\frac{1}{n}{\sum\limits_{n - 1}^{n}\; \frac{I({hkl})}{I_{0}({hkl})}}} \right\rbrack}^{- 1}$

-   (hkl)=measured intensity of the (hkl) reflection-   I₀ (hkl)=standard intensity of the standard powder diffraction data    according to JCPDF-card no. 42-1468-   n=number of reflections used in the calculation, whereby the (hkl)    reflections used are: (012), (104), (110), (113), (116), (300) and    (0 0 12).

It has surprisingly been found that improved cutting properties,improved chipping resistance and improved crater wear resistance of thecutting tool insert as well as lower friction in contact with theworkpiece can be achieved if the α-Al₂O₃ layer has a high sulphurcontent of more than 100 ppm analysed by Secondary Ion Mass Spectroscopy(SIMS) and a texture coefficient TC (0 0 12)>4 for the (0 0 12) growthdirection.

In a preferred embodiment of the present invention the at least oneα-Al₂O₃ layer of the cutting tool insert has a sulphur content of morethan 120 ppm, preferably more than 150 ppm analysed by SIMS. It has beenfound that the cutting properties of the inventive cutting tool can befurther improved by a higher sulphur content.

However, the sulphur content of the α-Al₂O₃ layer should not exceed 2000ppm, since a larger sulphur content may impair the properties of thecutting tool, such as grain boundary strength, and, in addition, causeporosity.

In another preferred embodiment of the cutting tool insert of thepresent invention the coating comprises, in addition to the at least oneα-Al₂O₃ layer, one or more refractory layers consisting of carbide,nitride, carbonitride, oxycarbonitride or borocarbonitride of one ormore of Ti, Zr, V and Hf, or combinations thereof deposited using CVD orMT-CVD, having a thickness of from 0.5 to 20 μm, preferably from 1 to 10μm.

Preferably, the coating comprises a first layer adjacent to thesubstrate body of CVD deposited Ti(C,N), TiN, TiC or HfN, or MT-CVDdeposited Ti(C,N), Zr(C,N), Ti(B,C,N), or combinations thereof. Mostpreferably, the first layer is if Ti(C,N).

In yet another preferred embodiment of the cutting tool insert of thepresent invention

a) the uppermost layer of the coating is the α-Al₂O₃ layer orb) the uppermost layer of the coating is a layer of carbide, nitride,carbonitride or oxycarbnitride of one or more of Ti, Zr, V and Hf, orcombinations thereof (herein called Ti top coating), having a thicknessof from 0.5 to 3 μm, preferably 0.5 to 1.5 μm, being deposited atop ofthe α-Al₂O₃ layer orc) surface areas of the cutting tool insert, preferably the rake face ofthe cutting tool insert, comprise the α-Al₂O₃ layer a) as the uppermostlayer whereas the remaining surface areas of the cutting tool insertcomprise as the uppermost layer a layer b) of carbide, nitride,carbonitride or oxycarbnitride of one or more of Ti, Zr, V and Hf, orcombinations thereof, having a thickness of from 0.5 to 3 μm, preferably0.5 to 1.5 μm, being deposited atop of the α-Al₂O₃ layer.

The Ti top coating layer atop the α-Al₂O₃ layer can be provided as awear indicator or as a layer of other functions. Embodiments, where onlyparts of the surface areas of the cutting tool insert, preferably therake face of the cutting tool insert, comprise the α-Al₂O₃ layer as theuppermost layer whereas the remaining surface areas are covered with theTi top coating as the outermost layer, can be produced by removing thedeposited Ti top coating by way of blasting or any other well knownmethod.

In another preferred embodiment of the cutting tool insert of thepresent invention the substrate consists of cemented carbide, preferablyof cemented carbide consisting of 4 to 12 wt-% Co, optionally 0.3-10wt-% cubic carbides of the metals from groups IVb, Vb and VIb of theperiodic table, preferably Ti, Nb, Ta or combinations thereof, andbalance WC.

For steel machining applications the cemented carbide substratepreferably contains 7.0 to 9.0 wt-% cubic carbides of the metals fromgroups IVb, Vb and VIb of the periodic table, preferably Ti, Nb and Ta,and for cast iron machining applications the cemented carbide substratepreferably contains 0.3 to 3.0 wt-% cubic carbides of the metals fromgroups IVb, Vb and VIb of the periodic table, preferably Ti, Nb and Ta.

In another preferred embodiment of the cutting tool insert of thepresent invention the substrate consists of cemented carbide comprisinga binder phase enriched surface zone having a thickness of 5 to 30 μm,preferably 10 to 25 μm, from the substrate surface, the binder phaseenriched surface zone having a Co content that is at least 1.5 timeshigher than in the core of the substrate and having a content of cubiccarbides that is less than 0.5 times the content of cubic carbides inthe core of the substrate. The thickness of the α-Al₂O₃ layer in thisembodiment is preferably about 4 to 12 μm, most preferably 4 to 8 μm.

Preferably, the binder phase enriched surface zone of the cementedcarbide body is essentially free from cubic carbides. The binderenriched surface zone enhances toughness of the substrate and widens theapplication range of the tool. Subtrates having a binder enrichedsurface zone are particularly preferred for cutting tool inserts formetal cutting operations in steel, whereas cutting tool inserts formetal cutting operations in cast iron are preferably produced withoutbinder enriched surface zone.

In another preferred embodiment of the cutting tool insert of thepresent invention the at least one α-Al₂O₃ layer has a texturecoefficient TC (0 0 12)>5, more preferably a texture coefficient TC (0 012)>6 for the (0 0 12) growth direction.

The present invention further provides a method of manufacturing acutting tool insert as defined herein wherein said at least one α-Al₂O₃layer is deposited by chemical vapour deposition (CVD) the reaction gasof the CVD process comprising H₂, CO₂, AlCl₃ and X, with X being H₂S,SO₂, SF₆, or combinations thereof, and optional additions of N₂ and Ar,wherein the X is present in the reaction gas mixture in an amount of atleast 1.0 vol-% of the total volume of gases in the CVD reaction chamberand wherein the volume ratio of CO₂ and X in the CVD reaction chamberlies within the range of 1≦CO₂/X≦7 during deposition of the at least oneα-Al₂O₃ layer.

It has surprisingly been found that the inventive α-Al₂O₃ coating can becontrolled by particular deposition conditions. The inventive kind ofhigh sulphur content and the texture coefficient TC(0 0 12)>4 of theα-Al₂O₃ coating can be achieved by the control of the volume portion ofthe sulfur containing dopant X in the reaction gas mixture of the totalvolume of gases in the CVD reaction chamber in an amount of at least 1.0vol-%, preferably at least 1.2 vol %, and, at the same time, by controlof the volume ratio of CO₂ and X in the CVD deposition reaction. Cuttingtests and friction tests have clearly confirmed the beneficial effectsof high sulfur content in the α-Al₂O₃ layer.

If the amount X is less than 1.0 vol-% of the total volume of gases inthe CVD reaction chamber the sulphur content and the texture coefficientTC(0 0 12) that can be achieved in the α-Al₂O₃ coating will not besufficiently high.

It has been found that the introduction of a high amount of sulphurcontaining dopant X alone will not lead to a high sulphur content andthe desired texture coefficient TC(0 0 12) in the coating. The inventorshave found that the ratio of sulfur containing dopant X to CO₂ duringCVD strongly affects the sulfur content and the texture coefficient TC(00 12) in the deposited α-Al₂O₃ layer. Studies by the inventors haveconfirmed that deposition of α-Al₂O₃ with a high sulfur content and thetexture coefficient TC(0 0 12) is difficult if too high CO₂/X ratiosduring deposition are used. It was surprising that the control of theCO₂/X ratio in the CVD deposition process of α-Al₂O₃ is the mostimportant factor to obtain a high sulfur content and the desired texturecoefficient TC(0 0 12) in the α-Al₂O₃ layer and, surprisingly and mostimportantly, that certain ratios resulted exclusively in high amounts ofsulfur with good reproducibly. Thus, the present invention provides fora new a method to control the sulfur content and the texture coefficientTC(0 0 12) of α-Al₂O₃ deposited by CVD.

In a preferred embodiment of the method of the present invention thevolume proportion of the component X or the combination of components Xis present in the reaction gas mixture during deposition of the at leastone α-Al₂O₃ layer in an amount of at least 1.2 vol-%, preferably atleast 1.5 vol-% of the total volume of gases in the CVD reactionchamber. In another embodiment the volume proportion of the component Xor the combination of components X lies within the range of 2.0 to 3.0vol-%.

It has surprisingly been found that the sulphur content and the texturecoefficient TC(0 0 12) of the α-Al₂O₃ layer can be further improved byhigher X content in the reaction gas mixture during deposition of theα-Al₂O₃ layer resulting in improved cutting properties, improvedchipping resistance and improved crater wear resistance of the cuttingtool insert. However, a too high content of X, for example above 5.0vol-%, should be avoided due to the danger of handling the sulphursources. For example, the preferred sulphur source, H₂S, is a flammableand extremely hazardous gas.

In another preferred embodiment of the method of the present inventionthe volume ratio of CO₂ and X in the CVD reaction chamber lies withinthe range of 1≦CO₂/X≦6 during deposition of the at least one α-Al₂O₃layer. When the deposition is carried out within this range of CO₂/X,both a sufficient amount of sulphur of >100 ppm in the alumina layertogether with a strong preferred growth of alumina along the (0 0 12)direction, resulting in a relatively high texture coefficient TC(0 0 12)for the α-Al₂O₃ layer, can be obtained.

In yet another preferred embodiment of the method of the presentinvention the volume ratio of CO₂/AlCl₃ in the CVD reaction chamber isequal or smaller than 1.5 and/or the volume ratio of AlCl₃/HCl in theCVD reaction chamber is equal or smaller than 1, during deposition ofthe at least one α-Al₂O₃ layer. If the ratio of CO₂/AlCl₃ is too high(>1.5) and/or if the ratio of AlCl₃/HCl is too high (>1.0),corresponding to too low amounts of HCl, this will enhance growth alongthe (0 1 2) direction and, consequently, will lead to a lower TC(0 0 12)in the resuiting alumina coating.

The CVD process of the present invention during deposition of the atleast one α-Al₂O₃ layer is suitably conducted at a temperature in therange of 850 to 1050° C., preferably 980 to 1050° C., most preferably1000 to 1020° C. If the temperature of the CVD process is too low, thegrowth rate would be too low, and f the temperature of the CVD processis too high, gas-phase nucleation and non-uniform growth will occur.

The reaction gas pressure range where the CVD process of the presentinvention is conducted during deposition of the at least one α-Al₂O₃layer is preferably from 50 to 120 mbar, more preferably from 50 to 100mbar.

In yet another preferred embodiment of the method of the presentinvention the component X in the CVD process is H₂S or SO₂ or acombination of H₂S and SO₂, whereby, if the component X in the CVDprocess is a combination of H₂S and SO₂, the volume proportion of SO₂does not exceed 20% of the volume amount of H₂S. If too much SO₂ is usedthe coating uniformity can be reduced due to the so-called dog-boneeffect.

In yet another preferred embodiment of the method of the presentinvention the reaction gas of the CVD process comprises additions of N₂and/or Ar in a volume amount in the range of 4 to 20 vol %, preferably10-15 vol %, of the total volume of gases in the CVD reaction chamber.

As will be shown in the examples below, the coatings of the inventionexhibit an excellent chipping resistance in a high-speed intermittentcutting and enhanced crater wear resistance in continuous turning overthe prior-art coatings.

Methods Secondary Ion Mass Spectroscopy (SIMS)

The measurement of sulphur in the alumina coatings has been done bySecondary Ion Mass Spectroscopy (SIMS) on a Cameca ims3f spectrometer.The quantitative determination of the sulphur concentration in a samplewas done relative to the known aluminum concentration in the sample. Forthe determination of the sensitivity (relative ion yield) for sulphurrelative to aluminum the reference glas SRM 610 of the NationalInstitute of Standards and Technology (NIST) was used. The sulphurconcentration in SRM 610 is 575 μg/g, and the relative accuracy of themeasurements is about ±20%.

The sample surface was sputtered with negative oxygen ions having anenergy of 14.5 keV. The primary ion current was about 30 nA, and thediameter of the focussed primary ion beam at the sample surface wasabout 30-40 μm. The generated positive secondary ions were acceleratedto an energy of 4.5 keV and measured with a mass spectrometer at a massresolution of m/Δm=1800 using a secondary ion multiplier in countingmodus (for³²S) and with a Faraday cup (for ²⁷Al), respectively. Thestarting energy of the detected secondary ions was 55±20 eV to lowermolecular interferences and increase the measurement accuracy (energyfiltering). As the measurement results the average of six equalmeasurement cycles was calculated. The integration times per cycle were25 sec for³²S and 3 sec for²⁷Al, respectively. For each sample themeasurements have been repeated 5 times.

TC(0 0 12) X-Ray Diffraction Measurements

X-ray diffraction measurements were done on a diffraktometer XRD3003PTSof GE Sensing and Inspection Technologies using Cu K_(α)-radiation. TheX-ray tube was run at 40 kV and 40 mA focussed to a point. A parallelbeam optic using a polycapillary collimating lens with a measuringaperture of fixed size was used on the primary side whereby theirradiated area of the sample was selected to avoid a spill over of theX-ray beam over the coated face of the sample. On the secondary side aSoller slit with a divergence of 0.4° and a 0.25 mm thick Ni K_(β)filter were used. θ-2θ scans within the angle range of 20°<2θ<100° withincrements of 0.25° have been conducted. The measurements were done on aflat face of the coated insert, preferably on the flank face. Themeasurements were done directly on the alumina layer as the outermostlayer. Any layer present in the coating above the alumina layer to bemeasured, if any, is removed by a method that does not substantiallyinfluence the XRD measurement results, e.g. etching. For the calculationof the texture coefficient TC(0 0 12) peak height intensities were used.Background subtraction and a parabolic peakfit with 5 measuring pointswere applied to the XRD raw data. No further corrections such as K_(α2)stripping or thin film correction were made.

CVD Coatings

All CVD coatings were prepared in a radial flow reactor, type Bernex BPX325S.

EXAMPLES Example 1 α-Al₂O₃ Coatings

Cemented carbide substrates for cutting inserts with a composition of6.0 wt % Co and balance WC (hardness about 1600 HV) were coated with aTi(C,N) layer by applying MT-CVD using 0.6 vol % CH₃CN, 3.8 vol % TiCl₄,20 vol % N₂ and balance H₂. The thickness of the Ti(C,N) MT-CVD layerwas about 5 μm.

Onto this Ti(C,N) layer of separate substrate samples different layersconsisting of about 8 μm α-Al₂O₃ were deposited. The coating parametersare given in Table 1, and the texture coefficients, TC(0 0 12), measuredby X-ray diffraction, and the sulphur concentrations in the α-Al₂O₃coatings, measured by SIMS, are given in Table 2.

The deposition of α-Al₂O₃ was started by depositing a 0.05 μm to about 1μm, preferably 0.5 μm to about 1 μm, thick bonding layer on top of theMTCVD layer from the system H₂—N₂—CO—TiCl₄—AlCl₃ at a pressure of 50 to100 mbar. For the preparation of the bonding layer the MTCVD layer wastreated with a gas mixture of 3 vol % TiCl₄, 0.5 vol % AlCl₃, 4.5 vol %CO, 30 vol % N₂ and balance H₂ for about 30 min at a temperature ofabout 1000° C. The deposition was followed by a purge of 10 min using H₂before starting the next step.

α-Al₂O₃ was nucleated on the (Ti,Al)(C,N,O) bonding layer by treatingsaid layer with a gas mixture of 4 vol % CO₂, 9 vol % CO, 25 vol % N₂,balance H₂ for 5-10 min at a temperature from about 750 to 1050° C.,preferably at about 980 to 1020° C. and most preferably at 1000 to 1020°C. (P=80 to 100 mbar). The oxidation was followed by a purge of 10 minusing Ar.

The alumina deposition was started with by introducing a gas mixture ofAlCl₃, CO₂, Ar₂, N₂ HCl and H₂, in the volume amounts as indicated intable 1, without precursor X for about 10 min at a temperature of about1000° C. These precursors were shunted in simultaneously except HCl. HClflow was shunted into the reactor 2 min after the start (8 min before Xwas introduced).

TABLE 1 α-Al₂O₃ coatings H₂S SO₂ CO₂ AlCl₃ Ar₂ N₂ HCl H₂ Pressure CO₂/XCoating [vol %] [vol %] [vol %] [vol %] [vol %] [vol %] [vol %] [vol %][mbar] ratio  1a 0.5 — 5.0 3.4 5.0 10.0 3.4 bal. 80 10  1b 0.5 — 2.5 3.25.0 10.0 3.3 bal. 80 5  2a 1.0 — 8.0 5.4 5.0 10.0 5.4 bal. 80 8  2b X1.0 — 4.0 2.7 5.0 10.0 2.7 bal. 80 4  3a 1.8 — 14.4 9.6 5.0 10.0 9.6bal. 80 8  3b X 1.8 — 9.0 6.0 5.0 10.0 6.0 bal. 80 5  3c X 1.8 — 3.6 2.45.0 10.0 2.4 bal. 80 2  4a 2.4 — 21.6 14.4 5.0 10.0 14.4 bal. 80 9  4b X2.4 — 2.4 1.6 5.0 10.0 1.6 bal. 80 1  5 0.4 0.1 4.0 2.7 5.0 10.0 2.7bal. 80 8  6 0.9 0.1 16.0 10.5 5.0 10.0 11.0 bal. 80 16  7 X 0.9 0.1 4.04.7 5.0 10.0 7.2 bal. 80 4  8 1.5 0.3 15.0 10.0 5.0 10.0 12.0 bal. 808.3  9 X 1.5 0.3 7.5 5.0 5.0 10.0 5.0 bal. 80 4.2 10 X 1.5 0.3 2.0 1.45.0 10.0 1.4 bal. 80 1.1 11 2.0 0.4 20.0 2.0 5.0 10.0 2.0 bal. 80 8.4 12X 2.0 0.4 3.0 2.0 5.0 10.0 2.0 bal. 80 1.3 X = invention

TABLE 2 α-Al₂O₃ coatings Sulphur Coating TC(0 0 12) [ppm]  1a — 15  1b2.2 30  2a 2.6 40  2b X 4.8 101  3a 3.8 60  3b X 4.1 108  3c X 5.9 220 4a 3.2 80  4b X 6.8 390  5 4.2 51  6 0.8 32  7 X 5.7 102  8 3.8 40  9 X6.1 119 10 X 6.7 222 11 2.3 72 12 X 6.9 385 X = invention

The inserts with coatings 4 a and 4 b were tested for frictioncoefficient using the pin-on-disc method. The coating 4 a showed afriction coefficient of 0.56, whereas the coatings 4 b and 12 showed alower friction coefficient of 0.42 and 0.39, respectively. Thus, a highsulphur content in the alpha alumina coatings has been identified to befriction reducing.

Example 2 Edge Toughness Tests

The samples 1 a to 4 b of Example 1 were tested with respect to edgetoughness (chipping resistance) in longitudinal turning of cast iron(GG25) using the following cutting parameters:

Work piece: GG25; cylindrical barInsert type: SNUNCutting speed: v_(c)=400 m/minFeed (f)=0.4 mm/revDepth of cut: a_(p)=2.0 mmRemarks: dry turning

The inserts were inspected after 2 and 4 minutes of cutting. As shown inTable 3, compared to the coating of the prior art, the edge toughness ofthe samples was considerably enhanced when the coating was producedaccording to this invention.

TABLE 3 Edge Toughness Flaking of the edge line (%) Flaking of the edgeline (%) Coating after 2 minutes after 4 minutes 1a 22 34 1b 18 32 2a 1841 2b 12 32 3a 19 29 3b X 2 8 3c X 0 4 4a 18 33 4b X 0 3 X = invention

Example 3 Turning Tests

The samples 6, 8, 10 and 12 of Example 1 were tested in carbon steel(C45) without coolant using the following cutting parameters:

Work piece: C45Insert type: WNMG080412-NM4Cutting speed: v_(c)=280 m/minFeed (f)=0.32 mm/revDepth of cut: a_(p)=2.5 mmRemarks: dry turning

The end of tool life criterion was flank wear >0.3 mm. Three edges ofeach variant were tested.

TABLE 4 Turning Test results Coating Tool Life (min)  6 13.2  8 15.5 10X 21.3 12 X 32.2 X = invention

1. A cutting tool insert consisting of: a substrate of cemented carbide,cermet, ceramics, steel or a superhard material; and a coating with atotal thickness of 5 to 40 μm, the coating consisting of one or morerefractory layers of which at least one layer is an α-Al₂O₃ layer havinga thickness of 1 to 25 μm, wherein the at least one α-Al₂O₃ layer has asulphur content of more than 100 ppm analysed by Secondary Ion MassSpectroscopy (SIMS) and the at least one α-Al₂O₃ layer has a texturecoefficient TC (0 0 12)>4 for the (0 0 12) growth direction, the TC (0 012) being defined as follows:${{TC}\; \left( {0\mspace{11mu} 0\mspace{11mu} 12} \right)} = {\frac{I\left( {0\mspace{11mu} 0\mspace{11mu} 12} \right)}{I_{0}\left( {0\mspace{11mu} 0\mspace{11mu} 12} \right)}\left\lbrack {\frac{1}{n}{\sum\limits_{n - 1}^{n}\; \frac{I({hkl})}{I_{0}({hkl})}}} \right\rbrack}^{- 1}$(hkl)=measured intensity of the (hkl) reflection I₀ (hkl)=standardintensity of the standard powder diffraction data according toJCPDF-card no. 42-1468 n=number of reflections used in the calculation,whereby the (hkl) reflections used are: (012), (104), (110), (113),(116), (300) and (0 0 12).
 2. The cutting tool insert of claim 1 whereinthe at least one α-Al₂O₃ layer has a sulphur content of more than 120ppm analysed by SIMS.
 3. The cutting tool insert of claim 1, wherein thecoating comprises, in addition to the at least one α-Al₂O₃ layer, one ormore refractory layers consisting of carbide, nitride, carbonitride,oxycarbonitride or borocarbonitride of one or more of Ti, Zr, V and Hf,or combinations thereof deposited using CVD or MT-CVD, having athickness of from 0.5 to 20 μm.
 4. The cutting tool insert of claim 1,wherein a) the uppermost layer of the coating is the α-Al₂O₃ layer or b)the uppermost layer of the coating is a layer of carbide, nitride,carbonitride or oxycarbnitride of one or more of Ti, Zr, V and Hf, orcombinations thereof, having a thickness of from 0.5 to 3 μm and beingdeposited atop of the α-Al₂O₃ layer or c) surface areas of the cuttingtool insert comprise the α-Al₂O₃ layer as the uppermost layer whereasthe remaining surface areas of the cutting tool insert comprise as theuppermost layer a layer of carbide, nitride, carbonitride oroxycarbnitride of one or more of Ti, Zr, V and Hf, or combinationsthereof, having a thickness of from 0.5 to 3 μm and being deposited atopof the α-Al₂O₃ layer.
 5. The cutting tool insert of claim 1 wherein thesubstrate consists of cemented carbide optionally 0.3-10 wt-% cubiccarbides of the metals from groups IVb, Vb and VIb of the periodic tableand balance WC.
 6. The cutting tool insert of claim 1 wherein thesubstrate consists of cemented carbide comprising a binder phaseenriched surface zone having a thickness of 5 to 30 μm from thesubstrate surface, the binder phase enriched surface zone having a Cocontent that is at least 1.5 times higher than in the core of thesubstrate and having a content of cubic carbides that is less than 0.5times the content of cubic carbides in the core of the substrate.
 7. Thecutting tool insert of claim 1 wherein the at least one α-Al₂O₃ layerhas a texture coefficient TC (0 0 12)>5, for the (0 0 12) growthdirection.
 8. A method of manufacturing a cutting tool insert of claim1, comprising: depositing said at least one α-Al₂O₃ layer by chemicalvapour deposition (CVD), wherein the reaction gas of the CVD processcomprises H₂, CO₂, AlCl₃ and X, with X being H₂S, SO₂, SF₆, orcombinations thereof, and optional additions of N₂ and Ar, wherein the Xis present in the reaction gas mixture in an amount of at least 1.0vol-% of the total volume of gases in the CVD reaction chamber, andwherein the volume ratio of CO₂ and X in the CVD reaction chamber lieswithin the range of 1≦CO₂/X≦7 during deposition of the at least oneα-Al₂O₃ layer.
 9. The method of claim 8, wherein the volume proportionof the component X or the combination of components X is present in thereaction gas mixture during deposition of the at least one α-Al₂O₃ layerin an amount of at least 1.2 vol-% of the total volume of gases in theCVD reaction chamber.
 10. The method of claim 8, wherein the volumeratio of CO₂ and X in the CVD reaction chamber lies within the range of2≦CO₂/X≦6 during deposition of the at least one α-Al₂O₃ layer.
 11. Themethod of any of claim 8, wherein the volume ratio of CO₂/AlCl₃ in theCVD reaction chamber is equal or smaller than 1.5 and/or the volumeratio of AlCl₃/HCl in the CVD reaction chamber is equal or smaller than1, during deposition of the at least one α-Al₂O₃ layer.
 12. The methodof claim 8, wherein the CVD process during deposition of the at leastone α-Al₂O₃ layer is conducted at a temperature in the range of 850 to1050° C. and/or the CVD process during deposition of the at least oneα-Al₂O₃ layer is conducted at a reaction gas pressure in the range 50 to120 mbar.
 13. The method of claim 8, wherein the component X in the CVDprocess is H₂S or SO₂ or a combination of H₂S and SO₂, whereby, if thecomponent X in the CVD process is a combination of H₂S and SO₂, thevolume proportion of SO₂ does not exceed 20% of the volume amount ofH₂S.
 14. The method of any of claim 8, wherein the reaction gas of theCVD process comprises additions of N₂ and/or Ar in a volume amount inthe range of 4 to 20 vol % of the total volume of gases in the CVDreaction chamber.
 15. The cutting tool insert of claim 2 wherein atleast one α-Al₂O₃ layer has a sulphur content of more than 150 ppmanalysed by SIMS.
 16. The cutting tool insert of claim 7 wherein the atleast one α-Al₂O₃ layer has a texture coefficient TC (0 0 12)>6 for the(0 0 12) growth direction.
 17. The method of claim 9, wherein the volumeproportion of the component X or the combination of components X ispresent in the reaction gas mixture during deposition of the at leastone α-Al₂O₃ layer in an amount of at least 1.5 vol-% of the total volumeof gases in the CVD reaction chamber.
 18. The method of claim 12,wherein the temperature is in the range of 980 to 1050° C.
 19. Themethod of claim 12, wherein the temperature is in the range of 1000 to1020° C.
 20. The method of claim 12, wherein the reaction gas pressureis in the range 50 to 150 mbar.